Separating transient leakage exposure from endpoint cancellation in fast transmon single-qubit gates

  1. Haoran Yang,
  2. Fudong Liu,
  3. Weilong Wang,
  4. Yangyang Fei,
  5. and Zheng Shan
Fast single-qubit gates on weakly anharmonic transmons are limited by leakage to noncomputational states, and standard mitigations such as DRAG (derivative removal by adiabatic gate)
act on the leakage amplitude at the end of the gate. We show that this endpoint amplitude and the transient leakage exposure accumulated during the gate are two distinct control objectives that can be assigned to separate modules. The endpoint is a single sample of the drive spectrum, |Λ̃ (η)|2; the exposure is a band integral about η and governs leakage under dephasing, and the spectral-null condition Λ̃ (η)=0 constrains only the former. We realize this split in a path–endpoint separation pulse (PESP): a path-shaping pulse suppresses the exposure, and a two-tone endpoint-cancellation pulse cancels the residual amplitude. For a 10 ns RX(π/2) gate at η/2π=0.2 GHz, in numerical simulations the path-shaping pulse reduces the dephasing exposure by ∼21% relative to cosine DRAG and the independently simulated Lindblad excess leakage by ∼20%, consistent with Pϕexcess≃γϕTP¯dephA, whereas matched-budget endpoint-only and spectral-null controls leave it essentially unchanged. The residual endpoint floor splits exactly into a |2⟩ back-action and a |3⟩ cascade, which the two tones cancel one-to-one, driving the floor at the path-exposure knee from ∼7×10−7 to ∼3×10−8 without perturbing the path. By separating transient exposure from endpoint leakage, PESP turns leakage suppression in fast weakly anharmonic gates into a modular, interpretable control problem: dephasing-induced leakage and the coherent residual error are reduced by separate, individually verifiable modules.

Tunable Nonlocal ZZ Interaction for Remote Controlled-Z Gates Between Distributed Fixed-Frequency Qubits

  1. Benzheng Yuan,
  2. Chaojie Zhang,
  3. Haoran He,
  4. Yangyang Fei,
  5. Chuanbing Han,
  6. Shuya Wang,
  7. Huihui Sun,
  8. Qing Mu,
  9. Bo Zhao,
  10. Fudong Liu,
  11. Weilong Wang,
  12. and Zheng Shan
Fault-tolerant quantum computing requires large-scale superconducting processors, yet monolithic architectures face increasing constraints from wiring density, crosstalk, and fabrication
yield. Modular superconducting platforms offer a scalable alternative, but achieving high-fidelity entangling gates between distant modules remains a central challenge, particularly for highly coherent fixed-frequency qubits. Here, we propose a distributed hardware architecture designed to overcome this bottleneck by employing a pair of double-transmon couplers (DTCs). By synchronously controlling the two DTCs stationed at opposite ends of a macroscopic cable, our scheme strongly suppresses residual static inter-module coupling while enabling on-demand activation of a non-local cross-Kerr interaction with an on/off ratio exceeding 106. Through comprehensive system-level numerical simulations incorporating realistic hardware parameters, we demonstrate that this mechanism can realize a remote controlled-Z (CZ) gate with a fidelity over 99.99\% between fixed-frequency transmons housed in separate packages interconnected by a 25 cm coaxial cable. These results establish a highly viable, hardware-efficient route toward high-performance distributed superconducting processors.